Hello Ted,
adding a Vlan to the set of Vlans mapped to an MST instance is possible but the impact is quite great.
MST switches consider themselves as part of the same MST region if:
region name matches
revision number matches
Vlans to MST instances mapping matches. The last check is performed by putting on the MST BPDU an hash of the table rather then the whole vlans to instances table.
So you need an extended time window to do the changes because during these changes you will see the effects of multiple MST regions
However, MST allows for provisioning of the whole space of Vlans 1-4094 even for Vlans that do not exist at the moment in your network. In other words you can divide the Vlan space 1-4094 in N subsets associated to your N MST instances. So when in the future you need a new vlan you just have to pick a vlan number that is already associated to instance that represents the topology you would like to use for the new vlan.
About how to make the changes
show spanning-tree mst configuration
shows the current MST configuration
the changes are performed in MST configuration context
Router(config)#
spanning-tree mst configuration
Router(config-mst)#
instance instance_id vlan vlan_range
see
http://www.cisco.com/en/US/docs/switches/lan/catalyst6500/ios/12.2SX/configuration/guide/spantree.html#wp1098388
You need to perform the changes on all switches that run MST at the end of the process you will have again a single MST region.
So the best thing to do is to provision additional ranges for each MST instance as doing all this process for a single Vlan is too much work. Remember that you can assign to MST instances also Vlans that are not existing in your network.
MST requires a change in the way we approach the network.
Hope to help
Giuseppe
